Tunable loop antenna circuits



@Sheets-Sheot 1 BY ATTORNEY INVEINTOR Filed Aug. 28. 1941 J A RANKIN TUNABLE LOOP ANTENNA CIRCUIT Feb. 22, 1944.

Feb. 22, 1944. .1. A. RANKIN TUNABLE LOOP ANTENNA CIRCUIT {Sheets-Sheet 2 Filed Aug. 28, 1941 Follow 31 amour INVENTOR K. M ATTORNEY Patented Feb. 22, 1944 TUNABLE Loor ANTENNA omonrrs John A. Rankin, Port Washington, N. Y., assignor to Radio Corporation ofAniericaya corporation of Delaware AppIicationA uguSt'ZS, 1941, Serial No, 408,601

' 9 Claims. (o1. 250-40)v My present invention relates to tunable loop antenna circuits, and more particularly to improved tuning arrangements for loop antennae.

As is well known, the inductance of a loop antenna depends upon the area enclosed by the' loop, the wire thereof and the number of turns. To secure maximum performance in a loop antenna, that is, to secure maximum signal voltage pick-up, the effective height of the loop and Q should be high. The effective height may be increasedby increasing the area. bounded by the loop turns, or the number of turns of wire on the loop may be increased. Of course, both of these expedients may be employed. Either of these expedients increases the loop inductance, and hence, makes it diificult to tune the loop overa desired frequency range. Further, the inherent distributed capacity is increased by these expedients, and this also tends to reduce the tuning range when tuning is ac complished by a variable tuning condenser. I In the case of broadcast receivers operating over a frequency range of 530 to 1700 kilocycles (110;), it becomes difficult to use a loop antenna of sufiicient inductance to give substantially maximum signal voltage pick-up and yet employ either inductance, or condenser, tuning over the required frequency range.

Accordingly, it is one of the main objects of my present invention to provide a loop antenna circuit which is maximized as to inductance thereby to approximate ideal signal voltage pickup conditions, and simultaneously to provide a tuningmeans for the circuit which is adapted to cover a desired frequency range which in the absence of the invention is unattainable with the aforesaid maximum inductance value.

Another important object of my invention is to improve the tuning range of the loop antenna circuit of a broadcast receiver while maintaining maximized inductance in the loop circuit; there being utilized an auxiliary inductance element in circuit with the loop and tuning reactance to insure the aforesaid characteristics.

Another object of my invention is to provide a tunable loop antenna circuit for a broadcast receiver, operating in a range of 530 to 1700 kc., wherein the tuning element is a variable permeability tuner in series with the loop; there being arranged in shunt relation tothe loop an adjustable inductance element which is variable in magnitude with the tuner variation, thereby to maintain maximized inductance in the loop without reduction of said frequency range. Anotherobiect of niy inventionis to provide aloop antenna adapted to be varied in tuning by a variable condenser arranged in shunt relation to the loop; there being utilized in [association with'the loop and variable condenser an adjustable inductance element which is varied concurrently with the variable condenser, thereby to maintain maximum loop inductance over a predetermined frequency range.

Still other objects of the invention are to improve generally the efficiency of tunable loop antenna circuits, and more especiallyto providev loop circuits for broadcast receivers which are reliable in operation and economically manuf actured and assembled.

1 The novel features which I believe tobe characteristic of my invention are set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and methodof operation will bestbe understood by reference tothe following description taken ,in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

- In the drawings: 7 a Figure lshows a circuit diagram of a tunable loop antenna embodying the invention,

Figure 2 schematically shows a modificatiom Figures 3 to '7, inclusive, show :further-modb fications, I a

Figure 8 shows the invention applied to atunable amplifier circuit. I

Referring now to the'acc'ompanying drawings, wherein like reference charactersin the different figures designate similar circuit elements, the numeral in Figure 1 designatesza loop antenna of a predetermined area and turns of wire. .Since the manner of constructing such loops is well known to those skilled in the art, it is merely necessary to point out that inybroadcast receivers operating in the 530 to 1700 kc.- band such loops are used as antennae. ,Considering'thetube 2 to be thefirst'signal amplifier, the high potential termina1';3: of the loop isflusually connected'tothe signal grid through .thecoil 5, assumingthat a soy-called permeability tuner is used to vary the receiver ;tuning. ;-Sueh a tuner;- as is verywell :known ;at'pre sent,-;come prises an inductance coil uhavingzfin adjustable core composed of comminutedsfmagnetic material held together :by a binder; The core ;6 is adjusted in po'sitio'npbyaipmeic anicalgdevice; shown as :the dotted line 15, 'Whi6h{ concurrently adjust's'fthecore'jlfl ronthecindu' tance tune t e following tuned amplifierstage. (Of course; all

, example, the core the tunable circuits, where permeability tuning is used, have the cores thereof adjusted by the common mechanical mechanism I.

The condenser 9, connected between the grid end of coil 5 and ground, resonates the coil 5 and loop I to the desired frequencies. Otherwise the circuit details of the amplifier 2 are shown as well known in the art. The terminal 4 is shown at ground potential. The auxiliary variable inductance II], also of the adjustable comminuted, or powdered, magnetic material core type, has its core II arranged for mechanical adjustment by the mechanical control I. The coil I is connected from terminal 3 to ground. That is to say, coil I0 is effectively in shunt with the loop I, while coil 5 is in series therewith. It is understood, of course, that when cores 6 and II are entirely within the coils thereof, then the inductances 5 and II] are maximum. When the cores are out of the coils, then the coil inductances are a minimum. This is true for all the coils whose cores are displaced by mechanism I.

Assume, first, that coil I0 is out of the circuit.

If the inductance of loop I is made high so as to have maximum signal voltage fed to the amplifier 2, it will be found that the tuning range of the input circuit of tube 2 will be substantially less than the 530 to 1700 kc. range desired. This is due to the fact that the inductance of loop I is in series with the inductance of coil 5, and, hence, the maximum to minimum inductance ratio of the circuit is much lower than that of coil 5 alone. The maximum to minimum inductance ratio of coil 5 should be of the order of :1 to cover a frequency range of the order of 3:1. It is found that using a high inductance loop in series with such a coil results in a reduction of the said 10:1 ratio to a value such that the frequency range is greatly reduced. According to my invention the coil I0 is shunted across the loop so as to provide an effective inductance for-loop I and coil III which is equal to the reciprocal of the sum of the reciprocals of L1 and L10. Hence, the effective inductance in series with tuner coil 5 is much less than the inductance of loop I at the different settings of tuner mechanism 1. Of course, the coil I0 may be constructed in any desired manner to secure any desired effect on the variation of the inductance of the tunable circuit. For II may be given various shapes, or the windings of coil I0 may be arranged to suit the designers purposes. If the tuning range is 530 to 1700 kc., loop I may be designed to have maximum signal voltage pick-up such as would, in the absence of coil I0, greatly reduce the range. Coil I0 is constructed and arranged so as to have its inductance vary in the same sense as the inductance variation of coil 5; the rate of variation of the inductance of coil I0 will depend upon the various factors which have to be satisfied. Physical dimensions for loop I, coil 5, coil I0 and condenser 9 are not given, because they are entirely dependent on the purposes to which the receiver is to be put. Design factors will usually limit the inductance of loop I to from 10 to 20 times the value that would be used in the absence of coil III.

In order to present a clear picture of my invention, the following explanation and illustrative examples are given: Let it be assumed that coil I0 is removed from the circuit, and that'the inductance of loop .I is microhenries (mh.). Further, let it be assumed that the tuning range is to be 3:1 in extent; for example, from 550 kc. to 1650 kc. If coil 5 has a minimum inductance of 120 mh. and a maximum inductance of 1200 mh., the maximum and minium of the entire circuit will be 1200 plus 15 for the maximum value, and 120 plus 15 for the minimum value. The ratio of 1215 mh. to 135 mh. equals 9:1. As the frequency range is equal to the square root of the inductance range, the

; resultant frequency range will be 3:1.

Now, if the loop inductance is increased to 150 mh., and the same coil is used at 5, the frequency range will be 1350 mh. divided by 270 mh. or /5:1. The resulting frequency range will then be reduced from 3:1, to or 2.24:1. However, if coil I0 is included as a fixed inductance, the effective inductance looking into the circuit comprising loop I and coil II] in shunt may be made to look like 15 mh., if coil I0 is given a value of 16.55 mh. This, however, results in a reduction in the voltage applied to the grid of tube 2. Hence, if coil III is made variable and with a higher inductance than 16.55 mh., the grid voltage obtained will likewise be greater. For example, coil I0 may be variable over a 10:1 range from 30 to 300 mh. The effective inductance looking into the loop and coil I0 in shunt will be 25 mh. when coil II) has minimum inductance, and mh. when coil ID has its maximum inductance value.

This means that if coil 5 is made to have a minimum inductance of mh. and a maximum inductance of 1100 mh. (10:1 range), the circuit minimum will be mh. while the maximum inductance of the circuit will be 1200 mh. The range of inductance of the circuit will then be 1200 divided by 135 or 8.9, and the frequency range will be substantially 3:1. Had a slightly lower value of minimum inductance for coil I0 been used, the tuning range would have been exactly 3:1. The advantage here is that at the high frequency end of the range the variable. coil II] has approximately twice as much inductance as the equivalent fixed coil I0, and at the low end of the frequency range the ratio of improvement is nearly twenty times. The coil inductances required do not have a greater range (10:1), but the improvement in gain is substantial.

In Figure 2 there is shown a modification of the manner of connecting auxiliary coil II] in the loop circuit. Here, the coil II] is connected in a closed series circuit with loop I and tuner coil '5. This arrangement is not as preferable as the arrangement of Figure 1, because with this arrangement the maximum loop inductance will be limited in value, and, hence, the voltage transferred to the grid circuit will be less.

- The invention is not restricted to use with a permeability tuner for loop I. In Figure 3 the loop I is shown shunted by a variable condenser I2 to tune the loop circuit over the desired broadcast range of 530-1700; kc. In this case the inherent distributed capacity across loop I, caused by increase of the inductance of the loop, shunts the tuning condenser I2. Thistends to reduce the frequency range of the loo-p circuit. Hence, auxiliary coil I0 is shunted across the loop and tuning condenser, and the coil is constructed in the same manner as shown in Figure 1. The mechanical adjustment device I has the core of coil I0 coupled thereto so as to adjust the inductance of coil III in the same sense as the variation of condenser It. In this case the coil I tends to maintain the frequency range of the circuit. This is due tothe fact that both the inductance and capacitance in the circuit are changed simultaneously.' As the resonantfrequency is proportional to the reciprocal" of the product of the inductance and ca 'aa'citance, if both are varied the range requiredo-f either is less.

As an example: To produce a frequency range of 3:1 ratio it is necessary'to have acapacity range of 9 to 1 with a fixed inductance; However, if the inductance is also varied'and the inductance variation maximum to minimum is 3:1

then with a capacity variation of 3:1, maximum 4 to minimum, the frequency range will be maintainedat'311, or the square root of the product of the ranges of'the inductance and capacity.

In Figure '3 the distributed capacity of the loop reduces the frequency range, because it is directly across the tuning condenser 12 and adds to the minimum circuit capacity. As an example: for a 3:1 tuning range the capacity range should be 9:1, and might be from 50 to 450 micromicrcfarads (mmf.). If the distributed capacity of the coil is 10 mini. the new maximum to minimum values would be 460 to '60, or a ratio of 7.66 to 1 with a resulting frequency range of 2.77 to 1. The above, of course, assumes a fixed coil in which, when made variable, overcomes this to restore the frequency range to the original 3: 1 ratio. To expand the practical example, let it be assumed that the tuning range offered by the condenser alone is 2.77 to 1 which is a capacity range of 7.66 to 1. If coil I0 is made of proper magnitude and variation so that the variation of inductance in the circuit is'1.1 75 to 1 then the tuning range of the system" will be which corresponds to a frequency range of 3:1. Quite obviouslyfor every value of loop inductance there will be required a different magnitude and variation ofco-illll to produce this 1.175 to 1 inductance change. The sense of variation of coil in is the same as that of capacitance i2.

In Figure 4 the variable condenser l2 not only has the auxiliary inductance coil H) in shunt relation therewith, but a second auxiliary inductance coil I0 is connected in a closed series circuit with loop I and coil It. The cores of Hi and N! are arranged for mechanical adjustment by adjusting means 1' of the variable condensers of the cascaded signal tuned circuits. In this circuit arrangement the coils Ill": and" 19 are chosen so that the entire circuit is resonant at the desired signal frequency even though the loop inductance might be extremely high and have high distributed capacity. The coils l0 and It! will have the same sense of variation as that of the tuning condenser. 1

There are situations wherein the type of circuit shown in Figure 4 one of the auxiliary coils is invariable. Thus, the coil 10 of Figure 4 may be replaced by coil 10" of constant value. In this modification the function of ID" as shown in Figure is to provide the correct matching inductance for the combination of loop I and inductance l0 so that capacitance I2 will resonate with inductance ID to the desired frequency range. Coil l0 acts to increase the tuning range when a loop of high inductance and high distributed capacitance is employed. The sense of variation of ID is the same as that of the capacitance l2.

' The 'modificationshown inFigure 6 differs from that shown in Figure 5 in that the invariable coil it" is connected directly in shunt with the loop I. The auxiliary coil in is arranged in a closed series circuit comprising coil l0 and H1 and variable condenser 12. In this circuit arrangement the adjustable core of coil I0 is mechanically adjustedby tuning mechanism 1 with the rotors of the variable condensers [2. The functions and circuit relations of coils l0 and are similar to those discussed for those of Figure 5. I a

In Figure '7 there is shown a modification of the arrangement of-Figure 4 wherein the vari able coil It is placed directly in shunt across loop 1, instead of being directly in shunt across the variable condenser I2 as shown in Figure 4. This modification differs from that shown in Figure 4 because it permits the'use of an extremely high loop inductance, and if the Q of coil I0 is high, the voltage transferred to the grid circuit will be high.

It will be noted that in the a-foredescribed circuits two or more reactance elements are varied simultaneously to obtain the desired frequency range, so that reactance elements of lower maximum to minimum range may be'u'sed, or by using a high ratio of maximum to minimum ratio of reactanceelements a large (high inductance, high distributed capacity) loop may be used to obtain the desired frequency range. This concept may, also, be applied to a tuned radio frequency-amplifier circuit to increase the tuning range thereof.

Forexample, in Figure 8 there is shown an amplifie'r 26, say in a broadcastreceiver operating in a range of "530 to 1700 kilocycles. The amplifier may be coupled to a following amplifier tube by a network which comprises a variable'tuning condenser 21 which has connected inshunt relation thereto a variable inductance element 22.

The variable inductance element 22 may be of the type shown in Figure 1, and, in that case, the

adjustablecore of coil 22 will be mechanically coupled with the uni-controlled "tuning mechanism 23 sothat it can vary its position with the rotors of the tuning condensers of the cascaded circuits. The inductance magnitude of coil 22 will be varied in the same sense as the capacity variation of condenser 2 I. By means of a circuit ar'rangementof this type the frequency range can be extended overa range up to and beyond 2,000 kc, while using the variable condensers normally employed forthe narrower frequency range.

To "explain the operation of thecircuit in Figure Sto a greater extent, the resonant frequency of 'aparallel tuned circuit is expressed by micwhere f is the resonant frequency and L and C are the inductance and capacitance respectively. If only one of the elements of L or C is changed, the frequency change is equal to the reciprocal of the square root of that change. For example, if C is changed 9: 1, then f is changed fiff, or 3:1. However, if both L and C are changed, then f is proportional to If the change in C is 9: 1, and the L change is9z1, 9x9 equals 81, so that 1 change equals the square root of 81 or 9:1. In other words, 1 is dependent on the square root of the product of L and C, and by changing both simultaneously the results are greater than by changing only one. This explains why coil 22 and condenser 2| are concurrently varied to extend the frequency range.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a loop antenna, a variable reactance element connected with the loop for tuning the same over a range of frequencies to be received, said loop antenna in order to increase its signal pick-up being so constructed as to have an inductance which greatly exceeds that which will resonate with said reactance element over said range of frequencies, at least one variable inductance element connected in circuit with said loop and operable to allow the reactance element to tune the antenna circuit over the mentioned frequency range, and means for simultaneously adjusting said reactance element and said inductance element in such relative senses that op eration over said range of frequencies is maintained and said increased signal pick-up is achieved.

2. In combination with a loop antenna, a first variable inductance connected with the loop for tuning the same over a range of frequencies to be received, said loop antenna in order to increase its signal pick-up being so constructed as to have an inductance which greatly exceeds that which will resonate with said first variable inductance over said range of frequencies, a second variable inductance connected in shunt with said loop and operable to allow the first inductance to tune the antenna circuit over the mentioned frequency range, and means for simultaneously adjusting said first and second inductances in such relative senses that operation over said range of frequencies is maintained and said increased signal pickup is achieved.

3. In combination with a loop antenna, a variable condenser connected in shunt with the loop for tuning the same over a range of frequencies to be received, said loop antenna in order to increase its signal pick-up being so constructed as to have an inductance which greatly exceeds that which will resonate with said variable tuning condenser over said range of frequencies, at least one variable inductance element connected in circuit with said loop and operable to allow the variable condenser to tune the antenna circuit over the mentioned frequency range, and means for simultaneously adjusting said variable condenser and said inductance element in such relative senses that operation over said range of frequencies is maintained and said increased signal pick-up is achieved.

4. A signal-collecting system for radio receivers tunable over a prescribed range of wave frequencies, comprising a loop antenna, a condenser and a variable-permeability type inductance arranged in series, said loop antenna being so constructed as to have a much too large inductance value to allow the variable-permeability type inductance alone to tune the circuit over said frequency range, and an inductive reactance shunting the loop antenna of such value as to allow the variable-permeability inductance to tune the antenna circuit over the mentioned frequency range.

5. A signal-collecting system as defined in claim 4 wherein the inductive reactance is variable and is so mechanically linked to the variable-permeability inductance as to be co -actuated therewith by the same means.

6. A signal-collecting system for radio receivers tunable over a prescribed range of wave frequencies, comprising a loop antenna, a variable condenser shunting said loop antenna, said loop antenna being so constructed as to have a much too large inductance value to allow the variable condenser alone to tune the circuit over said frequency range, and an inductive reactance connected in circuit with said loop and of such value as to allow the variable condenser to tune the antenna circuit over the mentioned frequency range.

7. A signal-collecting system as defined in claim 6 wherein the inductive reactance is connected in shunt with the loop antenna.

8. A signal-collecting system as defined in claim 6 wherein the inductive reactance is variable and is so mechanically linked to the variable condenser as to be co-actuated therewith by the same means.

9. A signal-collecting system for radio receivers tunable over a prescribed range of wave frequencies, comprising a loop antenna, a condenser and a variable-permeability type inductance arranged in series,- said loop antenna being so constructed as to have a much too large inductance value to allow the variable-permeability type inductance alone to tune the circuit over said frequency range, a second variable-permeability type inductance shunting the loop antenna of such value as to allow the first variable-permeability inductance to tune the antenna circuit over the mentioned frequency range, said inductances each being provided with an adjustable ferromagnetic core, and means for simultaneously adjusting said cores.

JOHN A. RANKIN. 

